The present invention relates to tires and to treads for tires incorporating diene elastomer compositions reinforced with a white or inorganic filler.
It relates in particular to the coupling agents used for coupling reinforcing inorganic fillers and diene elastomers in such tires or tire treads.
It is known that generally, in order to obtain the optimum reinforcement properties imparted by a filler in a tire, in particular in a tread of such a tire, the filler should be present in the elastomeric matrix in a final form which is both as finely divided as possible and distributed as homogenously as possible. Now, such conditions may only be obtained insofar as the filler has a very good ability firstly to be incorporated into the matrix during mixing with the elastomer and to disagglomerate, and secondly to be dispersed homogenously in this matrix.
It is fully known that carbon black has such abilities, which is generally not true of inorganic fillers, because, for reasons of mutual attraction, the inorganic filler particles have an irritating tendency to agglomerate together within the elastomeric matrix. These interactions have the harmful consequence of limiting the dispersion of the filler and hence the reinforcing properties to a substantially lower level than that which it would be theoretically possible to achieve if all the (inorganic filler/elastomer) bonds that could be created during the mixing operation were in fact obtained; these interactions furthermore tend to increase the consistency of the rubber compositions in the uncured state and therefore to make them more difficult to work (“processability”) than in the presence of carbon black.
Since fuel economies and the need to protect the environment have become priorities, it has however proved necessary to produce tires having reduced rolling resistance, without adversely affecting their wear resistance.
This has been made possible in particular due to the use, in tires or treads for these tires, of new rubber compositions reinforced with specific inorganic fillers referred to as “reinforcing” fillers, which are capable of rivalling a conventional tire-grade carbon black from the reinforcing point of view. At the same time, these compositions offer a lower hysteresis, which is synonymous with less rolling resistance for the tires comprising them, and also improved grip on wet, snow-covered or icy ground.
Tires or treads for tires incorporating rubber compositions reinforced with reinforcing inorganic fillers of the siliceous or aluminous type have in particular been described in patents or patent applications EP-A-501 227 or U.S. Pat. No. 5,227,425, EP-A-735 088 or U.S. Pat. No. 5,852,099, EP-A-810 258 or U.S. Pat. No. 5,900,449, WO 99/02590, WO 00/05300, WO 00/05301 and WO 02/10269.
Mention will be made in particular of documents EP-A-501 227 and EP-A-735 088, which disclose tires or tire treads incorporating diene rubber compositions reinforced with precipitated silicas of high dispersibility, having a low hysteresis and a distinctly reduced rolling resistance, without other properties, in particular those of grip, endurance and above all of wear resistance, being adversely affected. Tires or treads having such a compromise of contradictory properties are also described in application EP-A-810 258, with specific aluminas of high dispersibility as reinforcing inorganic fillers.
Although the use of these specific inorganic fillers as reinforcing fillers has reduced the difficulties of processing the rubber compositions that contain them, they are still more difficult to process than for rubber compositions filled conventionally with carbon black.
In particular, it is necessary to use a coupling agent, also referred to as bonding agent, the function of which is to provide the connection or bond between the surface of the particles of inorganic filler and the elastomer, while facilitating the dispersion of this inorganic filler within the elastomeric matrix.
It will be recalled that here that (inorganic filler/elastomer) “coupling agent” is to be understood to mean, in known manner, an agent capable of establishing a sufficient chemical and/or physical bond between the inorganic filler and the diene elastomer; such a coupling agent, which is at least bifunctional, has, for example, the simplified general formula “Y-A-X”, in which:                Y represents a functional group (“Y” function) which is capable of bonding physically and/or chemically with the inorganic filler, such a bond being able to be established, for example, between a silicon atom of the coupling agent and the surface hydroxyl (OH) groups of the inorganic filler (for example, surface silanols in the case of silica);        X represents a functional group (“X” function) which is capable of bonding physically and/or chemically with the diene elastomer, for example by means of a sulfur atom;        A represents a divalent group making it possible to link Y and X.        
The coupling agents must particularly not be confused with simple agents for covering the inorganic filler which, in known manner, may comprise the “Y” function which is active with respect to the inorganic filler but are devoid of the “X” function which is active with respect to the diene elastomer.
Coupling agents, in particular (silica/diene elastomer) coupling agents, have been described in a large number of documents, the best known being bifunctional organosilanes bearing alkoxyl functions (that is to say, by definition, “alkoxysilanes”) as “Y” functions and, as “X” functions, functions capable of reacting with the diene elastomer, such as, for example, sulfur functions (i.e. comprising sulfur).
Thus, it was proposed in patent applications FR-A-2 094 859 or GB-A-1 310 379 to use a mercaptoalkoxysilane coupling agent for manufacturing treads for tires. It was quickly shown, and is today well known, that such mercaptosilanes are capable of giving excellent silica/elastomer coupling properties, but that these coupling agents cannot be used industrially because of the very high reactivity of the thiol-type sulfur functions —SH (“X” functions), which very rapidly results, during the preparation of the rubber compositions, in an internal mixer, in premature vulcanization, this also being known as “scorching”, in very high viscosities in the uncured state, and, finally, in rubber compositions which are virtually impossible to work and process industrially. To illustrate this problem, mention may be made for example of documents FR-A-2 206 330, U.S. Pat. No. 3,873,489 and U.S. Pat. No. 4,002,594.
To overcome this drawback, it was then proposed to replace these mercaptoalkoxysilanes by polysulfurized alkoxysilanes, in particular bis-(alkoxysilylalkyl) polysulfides such as described in very large numbers of documents (see for example FR-A-2 149 339, FR-A-2 206 330, U.S. Pat. No. 3,842,111, U.S. Pat. No. 3,873,489 or U.S. Pat. No. 3,997,581).
Among all these polysulfides, mention must be made in particular of bis-(trialkoxysilylpropyl) polysulfides, most particularly bis-3-triethoxysilylpropyl tetrasulfide (abbreviated to TESPT).
It will be recalled that TESPT, of the formula [(C2H5O)3Si(CH2)3S2]2, is sold in particular by Degussa under the name Si69 (or X50S when it is supported to 50% by weight on carbon black), or alternatively by Osi Specialties under the name Silquest A1289 (in both cases, in the form of a commercial mixture of polysulfides Sx having an average value of x which is close to 4).
This polysulfurized alkoxysilane TESPT is today considered as being the product providing, for tires or tire treads reinforced by a reinforcing inorganic filler such as silica, the best compromise in terms of resistance to scorching, hysteresis and reinforcing ability. It is therefore the coupling agent of reference for the person skilled in the art for tires filled with silica of low rolling resistance, sometimes referred to as “Green Tires” because of the energy saving offered (or “energy-saving Green Tires”). To be convinced of this, it is enough to consult the very large numbers of publications relating to these tires based on silica and TESPT.
The elastomeric compositions based on reinforcing inorganic filler such as silica and polysulfurized alkoxysilane coupling agent such as TESPT however have the known drawback of having very substantially slower vulcanization kinetics, as a general rule by a factor of two to three, compared with the conventional compositions filled with carbon black.
It is known that the longer curing times which result adversely affect the industrial processing of tires or treads for tires based on reinforcing inorganic fillers.